1. Field of the Invention
The present invention relates to an image reading apparatus and image reading method which read a color image by moving a reading sensor using a raster scan scheme and light source switching scheme.
2. Description of the Related Art
Conventionally, an image reading apparatus irradiates an original document with light from a light source, accumulates the light reflected by the original document in a photoelectric transducer, and converts the accumulated charge amount into pixel data, thereby reading the original document. A color image reading apparatus of a light source switching scheme sequentially switches between light sources of red (R), green (G), and blue (B) and reads an image, thereby acquiring pixel data of respective color components. In addition, an image reading apparatus of a raster scan scheme generally reads an original document for each line using a reading sensor with a reading width equal to or longer than the width of the original document. An image reading apparatus of a sheet feed scheme generally reads an original document for each line while conveying the original document. An image reading apparatus of a flat bed type generally reads an original document for each line while scanning a reading sensor. There has also been known a method of moving a reading sensor, with a reading width shorter than the width of an original document, in a direction perpendicular to the conveyance direction of the original document and alternately repeating the reading and conveyance of the original document for each reading width of the reading sensor, thereby reading the original document (see Japanese Patent Laid-open No. 11-331504).
There has been proposed a method of alternately repeating the conveyance of an original document (or reading sensor) and the reading of R, G, and B color components for each line (see Japanese Patent Laid-open No. 7-143286).
However, an image reading apparatus of a flat bed type executes the so-called moving-document reading which alternately repeats the movement of an original document (or reading sensor) and the switching between light sources for each raster to read the original document. For example, as shown in FIG. 10, first, an original document (or reading sensor) is moved after irradiating it with light from an R light source and reading one line. Next, the original document (or reading sensor) is moved after irradiating it with light from a G light source and reading one line. Lastly, the original document (or reading sensor) is moved after irradiating it with light from a B light source and reading one line. A total of three lines are thus read to obtain color image data corresponding to one line in a direction perpendicular to the conveyance direction of the original document and one pixel in the conveyance direction of the original document. Referring to FIG. 10, a halftone portion indicates image data corresponding to one pixel. Since the original document (or reading sensor) always moves during the reading of three lines, the reading position of the original document changes for each color in practice, as shown in FIG. 10. In other words, different portions are irradiated with light beams from the R, G, and B light sources and read. This makes it impossible to acquire accurate color data and therefore causes color misalignment, leading to deterioration in the read image.
To solve the problem of this color misalignment, the above-described Japanese Patent Laid-open No. 7-143286 proposes a method of switching between R, G, and B light sources for each line and then moving an original document (or reading sensor) instead of the so-called moving-document reading. In other words, this patent reference proposes a method of conveying an original document (or reading sensor) by one line and then reading R, G, and B color components, and alternately repeating the conveyance and reading. Even though this method eliminates color misalignment in units of pixel, a delay and vibration attributed to a conveying mechanism have adverse influences on the operation because the conveyance of the original document (or reading sensor) and the reading of the original document are alternately repeated for each line. This may decrease the position accuracy, resulting in deterioration in the read image. As a matter of course, when a sufficiently long time is taken until the conveying mechanism stabilizes, any delay and vibration attributed to the conveying mechanism have no adverse influences on the operation. However, since a standby time is necessary for each reading of one line, the reading time may prolong. Also, since a mechanism which prevents any delay and vibration of the conveying mechanism must be newly provided, the entire conveying mechanism may become complicated and the cost may rise.
Japanese Patent Laid-open No. 11-331504 describes a method of reading an image with a band width of 128 pixels for each color. In a certain band region, an R component is read and data (R1) of 48 pixels on the lower side of data of 128 pixels are accumulated in a memory. A G component is read and data (G1) of 48 pixels on the lower side are similarly accumulated in the memory. A B component is read in the same way. Next, in a band region adjacent to the certain band region, an R component is read and data (R2) of 48 pixels on the upper side of data of 128 pixels are accumulated in the memory. A G component is read and data (G2) of 48 pixels on the upper side are similarly accumulated in the memory. A B component is read in the same way. An averaging process is performed for the R components using the data R1 and R2 accumulated in the memory. Likewise, the averaging process is performed for the G and B components using the data G1 and G2 and B1 and B2, respectively. However, this patent reference merely proposes averaging pixel data having undergone overlapping reading, in order to reduce band nonuniformity for each reading band due to the gap between an original document and a reading sensor. This patent reference has an arrangement different from that according to the present invention, and neither refers to nor solves the color misalignment to be solved by the present invention.